When posting an NTO (Nominal to Office) or CTO (Client to Office) transfer, there is a Tick For Bills box, which behaves as follows:
Ticked – The posting will hit your debtors control
Unticked - The posting will hit your Unbilled Disbursements control
For example - If you are posting a credit to a matters office account by either NTO or OCR and you tick the Tick For Bills box, and the item it is paying is not billed you will see this issue occur.
The simple remedy is to pick up the disbursement in a bill, this reduces the DPU balance and increases the unpaid bill balance giving you a net zero figure.
Also worth noting, if your Disbursement contains VAT you need to bill this, to enable you to allocate the VAT against the credit.